It is become more and more common for tenure to not be offered to professors for several reasons:
1. Tenure puts a long-term financial burden on the college and also guarantees some sort of medical benefits, whose cost has risen by over ten percent a year.
2. Tenure eliminates most of the flexibility for a college to undergo a sweeping reorganization. All of the tenured faculty will have a job (or get bought out) no matter whether their department still exists. This makes eliminating small, financially unsupported programs difficult.
3. Eliminating tenure forces professors to not relax after earning tenure and to continue meeting high scholarly standards. I know numerous cases of professors in economics who have not changed their teaching notes in decades, in spite of new theories...and this is much worse in some other disciplines.
4. State governments (in the case of public institutions) and/or university trustees are pushing to cut back on tenured positions to help increase accountability. There have been strong movements from outside groups (mostly from the political right) to get rid of the academic 'deadwood' and have leaner faculty structures. As these groups often push for part-time and distance education, adjunct faculty seem to be the perfect fit.
In the not-too-distant future, I would expect tenure to become a relic at most public and lesser-endowed private institutions. At the top private LACs, with their large endowments and tradition of treating faculty very well (and with trustees who are largely happy alumni), tenure should continue. Where budgets are a constant concern and trustees tend to be political appointments instead of alumni, tenure won't be around much longer.